The present invention relates to anti-fouling compositions, in particular for application to ships' bottoms and other surfaces to be immersed in water. The antifouling paints comprise a metal toxicant, e.g., cuprous or zinc oxide or an organotin compound, e.g, triphenyltin acetate or tributyltin oxide, dispersed in a matrix of synthetic resin, e.g., a vinyl chloride resin. Such compositions frequently include an activator for the toxicant, e.g., gum rosin.
The effectiveness of such compositions in inhibiting growth of slimes, algae, barnacles and other organisms depends upon progressive leaching from the composition of the toxic metal ion, leaving further metal available for continued toxic action, and in theory the composition sould continue to be effective until all the metal is exhausted.
It has been found, however, that the existing compositions cease to be effective before they should and we have discovered that this is due to the presence in the water of sulphur producing bacteria which convert copper or zinc oxide into the sulphide, a form in which it is no longer active against barnacles and the like. This is believed to be due to reduction by the bacteria to sulphide ions of sulphate ions present in sea water.